The ability to precisely locate optical elements relative to one another is of critical importance in the fabrication of micro-optical devices, since the alignment tolerances between elements are often specified in submicron dimensions. Typically, such elements may include an optical signal source, such as a laser, a detector, and an integrated or discrete waveguide, such as a fiber-optic or GRIN rod lens. Additionally, such elements may include a fiber amplifier, optical filter, modulator, grating, ball lens, or other components for conveying or modifying an optical beam. Micro-optical devices containing such components are crucial in existing applications such as optical communication and consumer opto-electronics, as well as applications currently being developed, such as optical computing.
Maintaining precise alignment among the optical elements may be conveniently provided by an optical microbench, such as a silicon optical bench. An optical microbench comprises three-dimensional structures having precisely defined surfaces onto which optical elements may be precisely positioned. One material well-suited for use as an optical microbench is single crystal silicon, because single crystal silicon may be etched anisotropically to yield three-dimensional structures having planar sidewalls formed by the precisely defined crystallographic planes of the silicon. For example, the {111} silicon plane is known to etch more slowly than the {100} or {110} planes with proper choice of etchant. Thus, structures may be formed comprising walls that are {111} planes by anisotropic etching.
Since the optical elements sit within the three-dimensional structures at a position below a top surface of the silicon substrate, a portion of the optical path often lies below the top surface of the substrate, within the volume of the substrate. Accordingly, passageways must be provided in the optical microbench between three-dimensional structures so that light may travel between the elements disposed in the associated three-dimensional structures. Hence, an optical microbench should contain three-dimensional structures that communicate with one another through structures such as a passageway.
While discrete, non-communicating, three-dimensional structures may be conveniently formed by an isotropic etching, etched structures which communicate with one another at particular geometries, such as a convex corner, pose significant problems for applications in which it is desirable to maintain the precise geometry defined by the crystallographic planes. For example, where two {111} planes intersect at a convex corner, the convex corner does not take the form of a straight line intersection between two planes, but rather rounds to create a rounded intersection between the two {111} planes. As etching continues to reach desired depth of the structure containing the {111} planes, the rounding of the corners can grow to such an extent that a substantial portion of the intersection between the two {111} planes is obliterated. Since the {111} planes are provided in the three-dimensional structures to form a planar surfaces against which optical elements may be precisely positioned, absence of a substantial portion of the {111} planes at the intersection can introduce a great deal of variability of the positioning of the elements at the intersection. Thus, the benefits provided by the crystallographic planes can be unacceptably diminished.
Traditionally, to avoid etching intersecting features, dicing saw cuts may be used. However, dicing saw cuts can be undesirable, because such cuts typically must extend across the entire substrate and may not conveniently be located at discrete locations within the substrate. Moreover, dicing saw cuts create debris which may be deposited across the substrate surface and lodge within the three-dimensional structures, which may interfere with the precise positioning of optical elements within such a structure.
Therefore, there remains a need in the art for optical microbench technology which permits three-dimensional structures having crystallographic planar surfaces to intersect with other surfaces, without degrading the crystallographic orientation of the intersected planar surfaces.